Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Oct 25 2023 20:05:16 FOUS30 KWBC 252005 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 404 PM EDT Wed Oct 25 2023 Day 1 Valid 16Z Wed Oct 25 2023 - 12Z Thu Oct 26 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS AND EASTERN KANSAS... ....16Z Update... Main adjustments this forecast cycle are the expansion of the Moderate further east across TX/OK with an inclusion of the DFW metroplex, as well as an addition of a Moderate risk over portions of eastern Kansas where heavy rainfall over the course of last night and this morning have lead to ongoing flooding south of Topeka. On these inclusions can be found below.... ....Central Texas into south central Oklahoma... Large scale forcing ahead of a deep shortwave trough centered over northern Chihuahua has aided in a line of heavy rainfall focused over west-central TX extending up into KS. Across the southern plains, rainfall rates have been observed consistently between 1-2" at peak intensity, but the slow forward progression of the main precip field has caused training of these heavier cells over portions of the area. Flash flood warnings have already been issued across the Concho Valley within the San Angelo urban area as observations have shown a general 1.5-3" of rain within the span of a few hrs. This is a trend that will continue to the east with increased convective coverage anticipated later this afternoon and evening with the initiation of the LLJ. Anomalous moisture field of 2-3 standard deviations above normal will persist across much of TX into OK which will enhance the heavy rainfall threat where convective impacts occur. 12z HREF probabilities are very high for neighborhood exceedance of 3" (70-80%) total, as well as 5" neighborhood exceedance (20-40%) across portions of central TX and southern OK. Upscale growth of the convective footprint will expand eastward overnight with hi-res guidance now depicting a greater impact into the DFW metro area which has a higher propensity for flooding due to the vast expanse of impervious surfaces within the urban corridor. Even the conservative EAS probability for exceeding 3" of rainfall has upwards of 20% across the Dallas metro and near 30% for the Fort Worth area. Rates of 1-2"/hr over several hrs will cause heightened flash flood concerns over the area, and with support from the Dallas/Fort Worth WFO, have elected to expand the Moderate further east to cover the northern sections of the I-35 corridor including the DFW metroplex. Rainfall totals of 5-8" with locally higher will be possible from the Edwards Plateau into north TX with much of the rainfall occurring within 3-5 hr intervals as the precip shield advances eastward. ....Eastern KS... A stalled frontal boundary over KS into MO with enhanced mid-level forcing and persistent southwesterly flow has allowed for a band of very heavy rainfall across portions of eastern KS, mainly northeast of Wichita and south of Topeka. Rainfall totals in excess of 8" have been reported across portions of Lyons county within the Topeka CWA forcing road closures out of Emporia. Another wave of moderate to heavy rainfall is forecast across the region later this afternoon and evening with the main axis of precip exiting after 06z. Considering the already saturated grounds within the corridor of expected rainfall, and with collaboration from the local Wichita/Topeka WFO's, have added a Moderate risk to the highly sensitive areas that were impacted overnight into this morning. Convective pattern will improve slightly once again as instability gradient that was pushed south from the area cold pool will return back the area of interest. 12z HREF probabilities for rainfall rates of 1"/hr or greater lie within the 25-30% range, but this is more than sufficient to cause problems for areas that cannot take much more rainfall. Embedded shortwave within the mean flow causing the secondary uptick will shift to the northeast with heavy rain potential focused over Kansas City and points north which allowed for the expansion of the SLGT risk as well from a previous update.=20=20 Kleebauer Day 2 Valid 12Z Thu Oct 26 2023 - 12Z Fri Oct 27 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHERN MIDWEST AND CENTRAL TEXAS... ....20Z Update... The two areas from the previous D2 issuance remain fairly similar, but have expanded each MRGL area slightly to account with the recent short term trends and associated probabilistic data from the 12z HREF. Have also added a SLGT risk across east TX to account for a strong signal for locally higher totals reaching 2-4" over central TX. Our potent shortwave over TX will eject northeast into the central plains, eventually crossing into the Midwest by the second half of the D2 period. The frontal boundary drapped over the northern Midwest will provide a focal point for area convective development along and south of the boundary where the theta-E gradient will be present. Scattered showers and thunderstorms over IA/MN will advance northeastward through the period, but an enhancement in the afternoon and evening hours is anticipated due to the increasing ascent provided by the shortwave presence. 12z HREF probability for 3"/12-hrs is running between 20-30% within an area that has seen a fair amount of rainfall very recently. The combination of prolonged moderate to locally heavy rainfall over a long period stretching from Thursday AM through the early evening will provide an opportunity for flash flooding concerns, especially within the zone extending from La Crosse to Green Bay where the lower FFGs exist from the previous convective event. Further east into the Great Lakes, locally heavy rain threat will continue over northern MI with the HREF blended mean and ensemble bias-corrected QPF fields indicating a local forecast maximum oriented off Lake Michigan near Traverse City to Gaylord. This area saw some of the higher impact rainfall over the past 24-48 hrs leading to much lower FFG's compared to normal. This allowed for the SLGT risk to be maintained within the zone to accompany the western expansion of the SLGT into the MN/IA/WI border. Across central TX, very anomalous moisture presence will persist due to prevailing southerly flow ahead of the mean trough to the west and the western flank of our mid-level ridge east of the Mississippi. PWAT indices between 2-2.3" will be common along and east of I-35 and south of I-20. Weak mid-level forcing advecting up from MX and the adjacent Gulf will meet a convergence point created from the area convection that will be decaying to the west. A sharp theta-E gradient will likely develop in-of the area northeast of Austin where FFGs are much lower due to the soil type and higher runoff potential. This will be the focal point of the convergence signal with plentiful agreement among guidance, especially the probability of at least 5"/12-hrs between 12-00z on the latest neighborhood HREF probabilistic signal (30-40%). The EAS probability is also fairly robust for at least 2"/24-hrs (25-30%) located over the aforementioned inflection point. The combination of probabilistic signals and favorable synoptic scale setup was good enough to add a new SLGT risk to the along and east of I-35 near Austin/New Braunfels. Kleebauer ....Upper Mississippi Valley into the Upper Lakes... The well defined front stretching from the Upper Mississippi Valley into the Upper Great Lakes will continue to be the focus for additional heavy rains day 2. The next round of convection moving along and to the north of this front that is expected to begin late day 1, will continue into the day 2 time period as surface low pressure moving along this front helps to maintain strong west southwesterly flow into this front. This will support potential for additional heavy rains from northern IA, across far southeast MN, central to southern WI into the northern L.P. of MI and the southern U.P. of MI. The model consensus for the day 2 qpf axis has shifted southward from previous runs and has resulted in less overlap of areas that have received heavy rains over the past 24 hours and the day 2 qpf axis. ,...Central Texas... Large scale dynamics will be weakening across the Southern Plains day 2 as the closed low ejecting northeastward day 1 continues to weaken as it presses into the Central Plains and Mid Mississippi Valley region. However, an axis of anomalous PW values ..2 to 3+ standard deviations above the mean... will persist across eastern TX. Model consensus is for moderate to heavy amounts in this anomalous PW axis. This expected precip will be mostly to the southeast of the expected day 1 heavy amounts. Oravec Day 3 Valid 12Z Fri Oct 27 2023 - 12Z Sat Oct 28 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS INTO THE LOWER ARKANSAS RIVER VALLEY... ....20Z Update... Only a minor change necessary to the D3 MRGL from previous issuance. Guidance is still keying on the frontal convergence axis extending from north-central TX into the Arklatex by the end of the period. PWAT anomalies beginning at 2 deviations above normal rising to 3 deviations is forecast through the period allowing for locally heavy rainfall to be the norm within any area convection. There is some indications that the northeastern extent of the QPF field could trend higher as the recent guidance shows an uptick of the precip footprint as you head into Arkansas. This is well-defined within the D4 ERO where a SLGT risk is now forecast across the Ozarks. There is potential for there to be an upgrade to a SLGT somewhere within the forecast MRGL with some hints being given now within the First-Guess fields. There is enough spread in the ensembles for where the heaviest rain will occur, so have maintained continuity for the time being. Kleebauer ....Northeast Texas, southeast Oklahoma into northwest Arkansas... The anomalous PW axis across eastern Texas on day 2 will begin to lift northeastward ahead of the strong cold front sinking southeastward through the Central to Southern Plains during day 3. This front is expected to slow its southeastward push toward the end of the day 3 period becoming nearly stationary from northeast Texas into northwestern Arkansas. Precipitation should become more active along and ahead of this front late Friday afternoon into the early hours of Saturday when well defined frontal convergence and favorable right entrance region jet dynamics enhance uvvs in the anomalous PW axis. The model consensus is for a narrow axis of potentially moderate to heavy totals along this front from northeast Texas into southeast Oklahoma and northwest Arkansas. There may be some overlap of day 1 heavy precip and day 3 amounts across portions of northeast Texas into southeast Oklahoma. The threat level was maintained as marginal from the old day 4, but may need to be upgraded to slight in later issuances depending upon where the observed heavy amounts on day 1 occur. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-xUhrTO2jeVkjCKYqYcBF86ENKUI06JHgSycuSplg0ZG= m9wcWipHsikNxdvrbTcjpQnjllD8SMhtEvpCzsNl_IkjzyQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-xUhrTO2jeVkjCKYqYcBF86ENKUI06JHgSycuSplg0ZG= m9wcWipHsikNxdvrbTcjpQnjllD8SMhtEvpCzsNlDeVMpmA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-xUhrTO2jeVkjCKYqYcBF86ENKUI06JHgSycuSplg0ZG= m9wcWipHsikNxdvrbTcjpQnjllD8SMhtEvpCzsNldhnyf6M$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .